Thermionic cathode



Patented May 29, 1934 PATENT OFFICE THERMIONIC CATHODE Erwin F. Lcwry, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania No Drawing.

Original application August 22,

1928, Serial No. 301,426. Divided and this application December 28, 1933, Serial No. 704,311

1 Claim.

This application is a division of my copending application for Thermionic cathodes Serial No. 361,426 filed August 22, 1928.

My invention relates to thermionic cathodes and more particularly to cathode materials adapted to be coated with electron-emissive oxides.

An object of my invention is to provide an inexpensive base-metal material for the cathodes of vacuum tubes or thermionic-discharge devices that has a lower specific resistance than the materials known under the trade names of Konel, Karma or Nichrome and a greater tensile strength, at operating temperatures, than nickel.

Another object of my invention is to provide a base metal for oxide-coated filaments that will not act injuriously, at elevated temperatures, with the electron-emissive oxides.

A further object of my invention is to provide vacuum tubes or thermionic-discharge devices, the cathode for which is composed of an inexpensive base metal having a lower specific resistance than the metals known under the tradenames of Konel, Karma or Niohrome and a cornparatively high tensile strength.

It has heretofore been customary to employ platinum or platinum alloys, such as platinumiridium or platinum-rhodium for producing the cathodes of vacuum tubes or electron-discharge devices which are utilized to rectify, amplify or detect electric currents. Such alloys, however, are expensive, and attempts have, therefore, been made to provide a base-metal filament, which will not act injuriously at an elevated temperature, with an oxide coating to increase the emission of electrons.

Base-metals, such as nickel, and alloys, such as Karma, Nichrome and especially Konel, which is described in my copending application, Serial No. 144,911, filed on October 28, 1926, have been found suitable for receiving tubes of some types. The tensile strength of nickel, however, is comparatively low, and, for certain types of receiving tubes, for example, the 171A, it is desirable to provide a base metal which has considerably less resistance than Konel, Karma or Nichrorne, so

that the cross section may be kept small and the filament heated to a sufficient temperature to cause electron emission for a definite current and ivoltage.

I have made the discovery that an alloy of nickel with a deoxidizing agent or a mixture of such agents such as titanium, silicon, Vanadium, manments, will meet all of the above requirements and is satisfactory as a base metal for oxidecoated cathodes requiring a high tensile strength and a comparatively low resistance.

An alloy of nickel with ferro-titanium has been found particularly desirable. The titanium not only acts as a deoxidizing agent but also serves to increase the strength of the alloy.

The amount of alloying material which is added to the nickel to form my improved alloy will depend somewhat on the resistance characteristics of the filament desired. As a rule, however, the alloying element or elements is added in amounts not greater than 25% of which amount not more than 10% will consist of one or more elements which are known in the art as deoxidizers.

Alloys of this type have considerably greater tensile strength at 900 C. than pure nickel, and filaments made from such alloys have the distinct advantage that, when coated with the oxides of the alkaline-earth metals, they furnish satisfactory electron emissions at temperatures approximately 100 C. less than the same emission can be secured from platinum or nickel coated with the same alkaline-earth-metal oxides. The electron emission compares favorably with filaments produced from Konel, which is described in my copending application, to which reference has been made. My improved alloy, however, has considerably less resistance than Konel, which is a desirable characteristic for cathodes to be utilized in tubes of certain types.

An alloy which I have found especially suitable for my purpose may be made by combining parts, by weight, of nickel, 7.5 parts, by weight, I of iron and 2.5 parts, by weight, of titanium, the iron and titanium being added in the form of ferro-titanium alloy.

The alloy may be prepared by any of the well known metallurgical processes, such as by melting the nickel and ferro-titanium together in an electric furnace, care being taken to prevent access of oxygen. The alloy, after being cast and forged, is rolled, swaged and drawn to a wire of a suitable size for the filament required. The wire is then provided with a coating consisting of one or more of the alkaline-earth-metal oxides in any desired manner, but I prefer to provide the coating by passing the filamentary material through an aqueous suspension of one or more of the alkaline-earth-metal carbonates, preferably, a mixture of the carbonates of barium and strontium. The coated filament is then passed through an electric furnace in an atmosphere of carbon dioxide which dries and bakes the coating,

causing it to adhere to the filament. This operation may be repeated until the desired weight of coating is applied to the filament. The carbonates may be converted to the oxides by passing an electric current through the filament, or this operation may be performed when the several elements of the tube are heated to drive out the occluded gas.

Alloys of the above described composition have exceptional characteristics which render them highly desirable as materials for producing filaments. A filament made from an alloy of the above described composition had a resistance of 28 to 29 ohms per mil-inch when operated at 25 watts per square inch, as compared to 22 ohms per mil-inch for a 10% platinum-iridium alloy and 40 ohms per mil-inch for Konel.

ERVVIN F. LOWRY. 

